Solid state lithium electrochemical cells are known in the art and typically consist of a lithium or lithium-based metal anode, a lithium-ion conducting solid electrolyte, and a cathode containing a lithium ion insertion electrode material. An insertion electrode material is capable as acting as a cathode by virtue of its ability to reversibly accommodate lithium ions physically inserted into its structure during discharge of the cell and subsequently removed therefrom during charging of the cell. Such insertion electrode materials (or intercalation compounds) include V.sub.2 O.sub.5, TiS.sub.2, V.sub.6 O.sub.13, LiCoO.sub.2 which have satisfactory specific energy densities of about 300-900 Wh kg.sup.-1 and mid-discharge voltages of about 2-3 volts.
Like other elements in the transition metal group including niobium and tantalum, vanadium forms numerous and frequently complicated compounds because of its variable valence. The four principle oxidation states of vanadium are 2+, 3+, 4+ and 5+, and it forms derivatives from more or less well defined radicals such as VO.sup.2+ and VO.sup.3+. However, the vanadium oxide solids possess nominal stoichiometries which indicate a mixture of vanadium oxidation states is present in certain solid phases of vanadium.
Solid lithium electrochemical cells using V.sub.6 O.sub.13 as the active cathode material are well studied. K. West et al., J. Power Sources, 14 (1985) 235-246, studied V.sub.6 O.sub.13 as a cathode material for lithium cells using polymeric electrolytes. They found that the lithium insertion reaction was reversible in the composition interval Li.sub.x V.sub.6 O.sub.13 [0.ltoreq.x.ltoreq.8]. The high stoichiometric energy density for the ultimate composition Li.sub.8 V.sub.6 O.sub.13, 890 W h/kg, is very favorable for battery applications. P.A. Christian et al., U.S. Pat. No. 4,228,226 suggest that lithiated vanadium oxides of the composition Li.sub.x VO.sub.2+y [0&lt;y.ltoreq.0.4] may be prepared chemically by treatment of VO.sub.2+y with n-butylithium in hexane. Christian et at. report that the unit cell parameters derived from X-ray powder diffraction data suggests that the compositions Li.sub.x V.sub.6 O.sub.13 have a structure very similar to that of the monoclinic V.sub.6 O.sub.13 i.e. VO.sub.2+y [0.1&lt; y&lt;0.2], prepared at higher temperature. The use of Li.sub.x VO.sub.2+y, chemically manufactured as aforesaid, as the positive electrode material in a cell, permits the manufacture of cells in the discharged state.
It has been reported in U.S. Pat. No. 4,228,226, the disclosure of which is incorporated herein by reference in its entirety, that vanadium oxides with nominal compositions close to V.sub.6 O.sub.13 i.e. oxides with the nominal stoichiometry range from VO.sub.2.05 to VO.sub.2.2 are readily prepared by the thermal decomposition of ammonium vanadate, NH.sub.4 VO.sub.3, at a controlled rate in an inert atmosphere such as argon or nitrogen, at a temperature of approximately 450.degree. C. Furthermore, the heat treatment of V.sub.6 O.sub.13 prepared in this manner can alter the lithium capacity of the material when used as a cathode active material in solid secondary lithium cells. It has also been observed that the morphology of vanadium oxide solids can affect the lithium capacity of the material under the same circumstances.
D. W. Murphy et at., J. Electrochemical Soc. 128 (1981) 2053, report the synthesis of V.sub.6 O.sub.13 and V.sub.6 O.sub.13+x [0&lt;X.ltoreq.0.5]. Stoichiometric amounts of V.sub.2 O.sub.5 and vanadium metal powder were intimately mixed and heated to 600.degree. C. in an evacuated quartz tube. The vanadium-oxygen stoichiometry was verified by TGA in an oxygen atmosphere. V.sub.6 O.sub.13+x [0&lt;X.ltoreq.0.5] was produced by ball milling vacuum dried NH.sub.4 VO.sub.3 and thermally decomposing the ammonium vanadate under an argon stream. The disclosure of D. W. Murphy et al. is incorporated herein by reference in its entirety.
Vanadium oxides V.sub.3 O.sub.7, V.sub.4 O.sub.9, V.sub.6 O.sub.13 and V.sub.6 O--+x[0.16.ltoreq.X.ltoreq.0.5] have been examined by Murphy et al., ibid., as cathode materials in ambient temperature non-aqueous secondary lithium cells. According to Murphy et al., the best cathode materials are V.sub.6 O.sub.13 and a slightly oxygen-rich V.sub.6 O.sub.13+x. Only the latter cathode materials consistently exhibited substantial capacities, good rechargability, and high charge potentials; and therefore made the best candidates for use as cathode active materials in non-aqueous lithium secondary batteries.
A related co-pending application describes the use and method of making V.sub.6 O.sub.14+x [0&lt;x.ltoreq.1.0], Ser. No. 08/184,087, filed Jan. 19, 1994, as Attorney Docket No. 1325 (028574-322) entitled "A CATHODE ACTIVE MATERIAL V.sub.6 O.sub.14+x [0&lt;x.ltoreq.1.0], AND SOLID SECONDARY LITHIUM CELLS BASED THEREON" which is incorporated herein by reference in its entirety.
The oxidation of V.sub.6 O.sub.13 in a controlled oxidation atmosphere at high temperature is difficult if the formation of V.sub.2 O.sub.5 is to be avoided. Oxidation at lower temperatures is preferable but requires the use of an appropriate solvent. Aqueous media can be used but a low boiling organic solvent would make drying the product simpler and be less likely to interfere with the lithium intercalation properties of the product.
It would be advantageous if a low temperature method for making V.sub.6 O.sub.13+x [0&lt;X.ltoreq.2.0] in an organic solvent could be found which provides control over the degree of non-stoichiometry in the product compound, i.e. controls the value of X.